This project is a continuation of studies designed to characterize the expression and function of protein tyrosine kinases (PTK) cloned from natural killer cells. Two PTK have been identified during this project. The first is a Janus kinase, JAK3, that has now been extensively characterized. The primary emphasis of the project is now on a kinase that has significant homology to the carboxyl-terminal Src kinase (Csk); the Csk homologous kinase, Chk (previously known as Lsk). Prior to the discovery of Chk, Csk was the only PTK known to specifically phosphorylate the conserved carboxyl-terminal tyrosine of Src family kinases and down regulate their catalytic activity. Csk is ubiquitously expressed, and not significantly regulated transcriptionally. Accordingly, it affects basic cellular functions such as transformation and adhesion and targeted disruption of the Csk locus is embryonically lethal. In contrast to Csk, we have shown Chk to be constitutively expressed only in neurons, NK cells, and regulated by stem cell factor in bone marrow cells. In addition, we have demonstrated inducible expression of Chk in both T cells and blood monocytes. In T cells Chk is expressed 48-72 hrs after T cell stimulation, but levels return to near baseline by days 4-6 suggesting a role in down-regulation of TCR function after an initial stimulus. In support of this notion over expression of Csk has been shown to inhibit TCR signaling and IL-2 production. However, over expression of Chk did not significantly inhibit Jurkat TCR signal transduction. Chk expressing cells demonstrated TCR-mediated tyrosine phosphorylation and calcium mobilization comparable to Chk negative cells. In fact, Chk expressing Jurkat clones produced more IL-2 upon stimulation with PHA than did control non-Chk expressing clones. The stark contrast between the actions of Chk and Csk in T cells, together with inducible Chk expression in monocytes, suggests that Chk regulates receptor systems other than the TCR. Ongoing studies are addressing the biochemical basis of the differences between Chk and Csk, and evaluating the affects of targeted disruption of the Chk gene in mice.